Nanopatterning

Techniques for fabricating on sub-micron length scales span a wide range, from sophisticated lithographic methods that have their origins in the semiconductor industry to more recent materials and chemical advances that rely on self-organization. For delineating patterns below 100 nm, several approaches have been proposed (and indeed demonstrated). These include nano-imprint lithography (including micro-contact printing, mold-assisted lithography, and hot embossing lithography), near-field optical lithography, direct patterning on a nanometer scale with scanning-probe microscopes, self-assembly of monolayers, pattern formation based on phase separation of polymers, etc. The search is on for nonphotolithographic methods that could provide technologically simpler and cheaper nanofabrication strategies. Some of these approaches are better suited for producing individual nano-structures for the investigation of nanometer-scale devices; the throughput is likely to always remain impracticably low for commercial application. Others such as nanoimprint lithography have the potential of high throughput due to parallel processing, do not require sophisticated tools, and allow nanoscale replication for data storage.

The natural length scales of polymer chains and their morphologies in the bulk, which lie in the nanometer domain, make polymers ideal building blocks for nanopatterning. Recent developments in the use of polymers for the fabrication of nanostructures via lithographic and self-assembling strategies have been reviewed.

Nanopatterning via Phase Separation of Polymers

Block copolymers of flexible, chemically incompatible, and dissimilar blocks can microphase separate into a variety of morphologies with nanometer scale dimensions. This self-assembly process is driven by an unfavorable mixing enthalpy and a small mixing entropy, while the covalent bond between the two blocks prevents macrophase separation. When the microphase separated morphology can be controlled and turned into a useful structure, phase separation of block copolymers can be a powerful tool for fabricating nanostructures without additional lithography and processing steps. In addition, to block copolymers copolymers, random copolymers comprising sufficiently dissimilar components based on size and chemical nature, for e.g., SSQ-MMA copolymer, SSQ/Polypropylene blend, or physical blends of hydrophobic and hydrophilic polymers driven by an underlying pattern of incompatibility have been shown to yield functional and arbitrary patterns. The use of surfactants in such blends may be used to accentuate the areas of different surface tension.

Soft lithographic approaches have been combined with surfactant and particulate templating procedures to create to create hierarchically ordered oxides. A recent report combines molecular scale, evaporation-induced self-assembly (EISA) of organically modified mesophases with macroscopic, evaporative printing procedures. This allows the rapid fabrication of hierarchical structures exhibiting form and function on multiple length scales and at multiple locations. The formulation of the “ink” in such hierarchial assembly strategies is complex and may use a variety of surfactants.

Self-Assembled Monolayer Systems (SAMS)

Polyelectrolytes are defined as materials for which the solution properties in solvents of high dielectric constant are governed by electrostatic interactions over distances larger than typical molecular dimensions. These materials are widely used in industrial applications as dispersants in aqueous media, flocculating agents to coagulate slurries and industrial wastes, for sizing in textile and paper manufacture, and as conditioning additives to drilling muds and soil to prevent abrasive damage. More recently, they have been applied in molecular self-assembly techniques for thin film deposition of electrically conducting polymers, conjugated polymers for light emitting devices, nanoparticles,and noncentrosymmetric-ordered second order nonlinear optical (NLO) devices.

The technique of Self-Assembled Monolayers or SAMs is an ingeniously simple, yet powerful nanoscale approach for the fabrication of functional supramolecular assemblies for various device applications. It involves the alternate adsorption of anionic and cationic polyelectrolytes onto a suitable substrate. Typically, only one of these is the active layer, while the other enables the composite multilayered film to be bound by electrostatic attraction. Alternatively, the oppositely charged polyelectrolytes may serve as a barrier for the sustained release of an active core. Controlled formation of highly ordered, three-dimensional, multifunctional, reactive, thin films containing biological molecules is seeing widespread application in the areas of biotechnology and biomaterials science.

Of the potential polyanionic candidates, poly(styrenesulfonic acid) (PSSA) and its salts have been used extensively. Why? Excellent adsorption properties, water solubility, smooth films with easily controllable thickness, controlled level of loading/penetration of active component—all properties essential in realizing high performance devices based on supramolecular structures in terms of selectivity, sensitivity, response time, and stability. In addition, PSSA has been used to dope thiophene-based conjugated polymers to make them conducting, e.g., PEDOT/PSS Aldrich products. Post treatment of polyaniline (emeraldine salt), grafted to lignin (see Aldrich products , with PSSA may also be used to enhance electrical conductivity.

Norbornadiene

A scan of the scientific and patent literature reveals that this versatile monomer has been applied over a vast spectrum of high technology applications, in fields ranging from materials science, ag-related, and pharmaceutical, to being used as a model system in fundamental research activity (for example, in testing new nanocatalysts – in single-step hydrogenations, tandem cycloaddition reactions, Pauson-Khand annelation as well as in comprehensive theoretical studies. The unique set of properties offered by Norbornadiene along with its transformations driving some of the reported applications are highlighted in the following table.

Scheme 1. Conversion between norbornadiene (NBD) and quadricylane (QC).

Table 1. Properties of norbornadiene.

Imprinting, or embossing, is a well-known technique to generate microstructures in hard polymers by pressing a rigid master containing surface-relief features into a thin thermoplastic polymer film that is then heated close to or, more generally, above the Tg (see Figure 2). Nanoimprint lithography (NIL) has the potential of high-throughput due to the parallel processing, does not require sophisticated tools, and allows nanoscale replication for data storage.  NIL is also compatible with conventional device processing techniques. The quality of the nanoimprinting process depends on a number of experimental parameters like T, viscosity in the melt, adhesion of the polymer to the mold, etc. PMMA has been most widely used as the imprintable material, but a range of thermoplastic and thermosetting polymers is under investigation to optimize the imprinting and subsequent etching steps.

Figure 2. Schematic overview of nanoimprint lithography.

The rigid master is usually prepared via e-beam lithography and has feature sizes in the 10–100 nm size range. After imprinting the polymer film, further etching can transfer the pattern into the underlying substrate. Alternatively, metal evaporation and lift-off of the polymer mask produces nanopattern metal features.

Soft Lithography

Nanoimprint lithography (NIL) has primarily been used to emboss hard thermoplastic polymers. The micromolding and embossing of elastomers has attracted considerable interest as these materials have found important applications in softlithographic techniques such as microcontact printing (mCP). In this technique, a monolayer of a material is printed off an elastomeric stamp [made of poly(dimethylsiloxane) (PDMS)] after forming conformal contact between stamp and substrate (Figure 3). Sub-micron surface relief structures can easily be introduced in PDMS by curing the polymers against a lithographically prepared master. The advantage of mCP is the ability to pattern surfaces chemically at the sub-micron level.

Figure 3. Schematic overview of microcontact printing (mCP). (Images courtesy of Hongwei Li, Wilhelm T. S. Huck; University of Cambridge , Department of Chemistry , Melville Laboratory for Polymer Synthesis.)

An elastomeric stamp is inked with small molecules (thiols or silanes) and pressed against a clean substrate (gold or silicon wafer). Where the stamp is in contact with the surface, a monolayer of material is transferred to the substrate. A second thiol or silane is used to fill in the background to provide a chemically patterned surface.

Photochemical Acid Generators

Photoacid generators (or PAGs) are cationic photoinitiators. A photoinitiator is a compound especially added to a formulation to convert absorbed light energy, UV or visible light, into chemical energy in the form of initiating species, viz., free radicals or cations. Cationic photoinitiators are used extensively in optical lithography. The ability of some types of cationic photoinitiators to serve as latent photochemical sources of very strong protonic or Lewis acids is the basis for their use in photoimaging applications. The continuing decrease in device dimensions in the microelectronics industry is being achieved by pushing the limits of optical lithography. In chemically amplified resist technology, the radiation-sensitive material (resist) in which patterns are delineated typically includes a matrix polymer and an onium salt photoacid generator (or PAG). There are several materials’ issues to be considered in the choice of the PAG: sufficient radiation sensitivity to ensure adequate acid generation for good resist sensitivity, absence of metallic elements, temperature stability, etc.

The usual photo-supplied catalyst has been strong acid. Triarylsulfonium and diaryliodonium salts have become the standard PAG ingredients in CA resist formulations, because of their generally easy synthesis, thermal stability, high quantum yield for acid (and also radical) generation, and the strength and nonvolatility of the acids they supply. Simple onium salts are directly sensitive to DUV, X-ray and electron radiations, and can be structurally tailored, or mixed with photosensitizers, to also perform well at mid-UV and longer wavelengths. However, onium salts are ionic and many will phaseseparate from some apolar polymers, or not dissolve completely in some casting solvents. Nonionic PAGs such as phloroglucinyl and o,o-dinitrobenzyl sulfonates, benzylsulfones and some 1,1,1-trihalides are more compatible with hydrophobic media in general, although their thermal stabilities and quantum yields for acid generation are often lower.

The phenomenal rate of increase in the integration density of silicon chips has been sustained in large part by advances in optical lithography – the process, as described above, that patterns and guides the fabrication of the component semiconductor devices and circuitry. Although the introduction of shorter-wavelength light sources and resolution enhancement techniques should help maintain the current rate of device miniaturization for several more years, a point will be reached where optical lithography can no longer attain the required feature sizes. Several alternative lithographic techniques under development have the capability to overcome these resolution limits – EUV, X-ray, electron beam and ion beam lithographies, but, at present, no obvious successor to optical lithography has emerged.

The next 10 years promises to be an exciting period in the history of computers and networks as nanotechnology takes off to redefine a new level in the way computers are manufactured. It’s not entirely radical as the Lithographic principles behind the manufacturing process can be adopted for nanotech processes. What is revolutionary are the minute molecular-level sizes at which those circuit boards can now be made. This is the core of nanotech Continue reading »

The health-conscious buyers nowadays are trying to get more privy with their food choices including whole grains, fruits and vegetables. The recent whole grains campaign have led to a myriad of questions on where and how a certain produce is made. Even the way foods are shipped into the local super mart becomes just as important as its nutritional value.

There have already been debates about “http://en.wikipedia.org/wiki/Genetically_modified_organism” target=”_blank”>GMO or genetically-modified crops and many are raising their eyebrows on the advent of nanotechnology. There is not enough study made on this area and many still fear the hidden risks in using this new method in making smart foods.

You might not have noticed but nanotechnology is present in some of the foods you eat. These are tiny particles which can extend your food’s shelf life, enhance its colors and flavors, add greater nutritional values and sense bacterial contamination.

Nanotechnology is the latest player in the food industry, reconstructing your food at the atomic level. This process is so powerful that it can alter food products into hundreds of possibilities that transcends physical laws.

Here are the Top Reasons How Nanotech can Help in Giving More Value to Your Grains, Fruits and Veggies:

Better Nutrients

Nano-nutrients or particles are totally soluble, creating an opportunity for nutritional drinks which are equal or better than its whole food counterpart. This also means better absorption of essential vitamins, antioxidants, phytonutrients and minerals to every cell of your body. You only get the best superfood solution for maximum health.

Contaminant-Free

Studies show that nanotech aids in catching pathogens in your food and getting rid of it for good. When many people are suffering the woes of Salmonella in food, nanotechnology helps make these foods safe to consume. The nanoparticles can also be used to identify bacteria and get rid of it.  This means having a peace of mind from all those nasty microbes in your dish.

Better Food Packaging and Storage

Those nano-sensors in your food can help determine bacteria and spoilage. Nanotech can make an edible food film for perishable goods, which allows it to have a better shelf life. This means getting your food as fresh as it can be, or even better.

Pesticide-Free

Unlike GMO crops which are laced with copious amounts of pesticides, nanotech can reduce the impact of these harmful substances on food by creating better ways to enhance organic farming.

Better Flavor

Shrinking your favorite grains, fruits and veggies into nano crystals means getting a compact version of your whole foods, without sacrificing the healthful benefits. Designing smart foods that fit every consumer’s taste is a dream come true for many. For example, calcium particles can be added for those suffering osteoporosis.

Eco-Friendly

All those plant-based biodegradable plastics have been made, thanks to nanotechnology. This means seeing an advent in natural polymers that will take sustainable living into its novel meaning.

Just because we don’t understand how a process works means that we have to shun it, for good. With the ever-increasing need to sustain the growing populace, nanotech might just be that miracle solution for our demanding lifestyle of staying healthy and fit.

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Nanotechnology: Giving a new dimension to Food Industry

INTRODUCTION:

A derivative of chemistry, engineering, and physics, and micro fabrication techniques, nanotechnology involves manipulating matter at the nanoscale level. It is responsible for determining not only that biological and nonbiological structures measuring less than 100 nm exist but al Continue reading »